Heat-sensitive lithographic printing plate precursor

ABSTRACT

A heat-sensitive lithographic printing plate precursor is disclosed which comprises a hydrophilic support and a coating provided thereon, wherein the coating comprises an infrared absorbing dye and is optimised for producing a minimum extent of ablation when exposed to high power infrared laser light.

FIELD OF THE INVENTION

The present invention relates to a heat-sensitive lithographic printingplate precursor which requires alkaline processing.

BACKGROUND OF THE INVENTION

Lithographic printing typically involves the use of a so-called printingmaster such as a printing plate which is mounted on a cylinder of arotary printing press. The master carries a lithographic image on itssurface and a print is obtained by applying ink to said image and thentransferring the ink from the master onto a receiver material, which istypically paper. In conventional lithographic printing, ink as well asan aqueous fountain solution (also called dampening liquid) are suppliedto the lithographic image which consists of oleophilic (or hydrophobic,i.e. ink-accepting, water-repelling) areas as well as hydrophilic (oroleophobic, i.e. water-accepting, ink-repelling) areas. In so-calleddriographic printing, the lithographic image consists of ink-acceptingand ink-adhesive (ink-repelling) areas and during driographic printing,only ink is supplied to the master. Printing masters are generallyobtained by the so-called computer-to-film method wherein variouspre-press steps such as typeface selection, scanning, color separation,screening, trapping, layout and imposition are accomplished digitallyand each color selection is transferred to a graphic arts film using animage setter. After processing, the film can be used as a master.

A typical photosensitive printing plate precursor for computer-to-filmmethods comprises a hydrophilic support and an image-recording layerwhich includes UV-sensitive compositions. Upon image-wise exposure of anegative-working plate, typically by means of a film mask in a UVcontact frame, the exposed image areas become insoluble and theunexposed areas remain soluble in an aqueous alkaline developer. Theplate is then processed with the developer to remove the diazonium saltor diazo resin in the unexposed areas. So the exposed areas define theimage areas (printing areas) of the printing master, and such printingplate precursors are therefore called ‘negative working’. Also positiveworking materials, wherein the exposed areas define the non-printingareas, are known, e.g. plates having a novolac/naphtoquinone-diazidecoating which dissolves in the developer only at exposed areas.

In addition to the above photosensitive materials, also heat-sensitiveprinting plate precursors have become very popular. Such thermalmaterials offer the advantage of daylight stability and are especiallyused in the so-called computer-to-plate method wherein the plateprecursor is directly exposed, i.e. without the use of a film mask.Usually the material is image-wise exposed to heat or to infrared lightand the generated heat triggers a (physico-)chemical process, such asablation, polymerization, insolubilization by cross-linking of a polymeror by particle coagulation of a thermoplastic polymer latex, andsolubilization by the destruction of intermolecular interactions or byincreasing the penetrability of a development barrier layer.

EP 1 188 797 discloses a near-infrared absorbing material comprising anovel polymethine compound which shows a high sensitivity to a YAG laserhaving an emission wavelength of 900 nm to 1100 nm, and a printing plateutilizing said near-infrared absorbing material.

EP 1 096 315 discloses a negative-working printing plate precursorincluding a support and a photosensitive layer containing an infraredabsorber, an onium salt, a radical polymerizing compound and a binder.The absorbance of said photosensitive layer varies between 0.5 and 1.2.

EP 1 129 845 discloses a heat-mode printing plate comprising a on ahydrophilic support a photosensitive layer comprising an infraredabsorber, a polymerization initiator and a compound having apolymerizable unsaturated group in a solvent, wherein the residualsolvent in the photosensitive layer is 5% wt or less relative to theweight of the photosensitive layer.

Thermal plates which require no processing are also known; such platesare typically of the so-called ablative type, i.e. the differentiationbetween hydrophilic and oleophilic areas is produced by heat-inducedablation of one or more layers of the coating, so that at exposed areasa surface is revealed which has a different affinity towards ink orfountain than the surface of the unexposed coating. A major problemassociated with ablative plates, however, is the generation of ablationdebris which may contaminate the electronics and optics of the exposuredevice and which needs to be removed from the plate by wiping it with acleaning solvent, so that ablative plates are often not trulyprocessless. Ablation debris which is deposited onto the plate's surfacemay also interfere during the printing process.

Thermal plates are generally exposed to infrared light in aplate-setter, which can be of the internal drum (ITD), external drum(XTD) or flatbed type. The availability of low-cost, high-power infraredlaser diodes enables to manufacture plate-setters wherein thermal platematerials can be exposed at a higher drum rotation speed, resulting in ashorter total exposure time and a higher plate throughput. The highpower infrared laser diodes are able to provide a high power density(kW/cm²) at the plate surface resulting in the necessary amount ofenergy (J/cm²) in a shorter pixel dwell time. It is observed that such ahigh power exposure of so-called non-ablative thermal plates, i.e.plates which are not designed to form an image by ablation, neverthelessproduces partial ablation of the coating. This phenomenon is to beavoided in view of the problems associated with the generation ofablation debris as discussed above.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a thermallithographic printing plate precursor wherein the coating is optimisedfor producing a minimum extent of ablation when exposed to high powerinfrared laser light. This object is realized by the material of claim1. Specific embodiments of the invention are defined in the dependentclaims.

According to the method of the present invention, as defined in claim12, the printing plate precursor can be exposed to laser light having awavelength in the range of λmax±20 nm and a power density above 233kW/cm² without generation of ablation debris.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the infrared absorption spectrum of a comparative material(dotted line) and a material according to the present invention (solidline).

FIG. 2-6 are scanning electron microscopy (SEM) images of the coating ofa prior art material exposed to infrared laser light at various powerdensity values.

DETAILED DESCRIPTION OF THE INVENTION

The heat-sensitive lithographic printing plate precursor of the presentinvention contains a hydrophilic support and a coating provided thereoncomprising an infrared light absorbing dye and a hydrophobic binderwhich is soluble in an aqueous alkaline developer. The coating mayconsist of one or more layer(s). Examples of additional layers besidesthe layer(s) which comprise the hydrophobic binder or the layer(s) whichcomprise the infrared dye are discussed below.

The formation of the lithographic image by the printing plate precursorof the present invention is due to a heat-induced solubilitydifferential of the coating during processing in the developer. Thesolubility differentiation between image (printing, oleophilic) andnon-image (non-printing, hydrophilic) areas of the lithographic image ischaracterized by a kinetic rather than a thermodynamic effect, i.e. thenon-image areas are characterized by a faster dissolution in thedeveloper than the image-areas. In a most preferred embodiment, thenon-image areas of the coating dissolve completely in the developerbefore the image areas are attacked so that the latter are characterizedby sharp edges and high ink-acceptance. The time difference betweencompletion of the dissolution of the non-image areas and the onset ofthe dissolution of the image areas is preferably longer than 10 seconds,more preferably longer than 20 seconds and most preferably longer than60 seconds, thereby offering a wide development latitude.

The printing plate precursor is positive-working when after exposure byheat or infrared light and development the exposed areas of the coatingare removed from the support due to a higher dissolution rate in theaqueous alkaline developer than the unexposed areas and definehydrophilic (non-printing) areas, whereas the unexposed coating is notremoved from the support and defines an oleophilic (printing) area. Fora negative working printing plate precursor, after image wise exposureby heat or infrared light the is exposed image areas dissolve slower inan aqueous alkaline developer than the unexposed areas which remainsoluble. For the latter plate precursor the exposed areas define theimage areas or printing areas. The printing plate precursor of thepresent invention can be positive- or negative-working with the positiveworking embodiment being preferred.

The support of the lithographic printing plate precursor has ahydrophilic surface or is provided with a hydrophilic layer. The supportmay be a sheet-like material such as a plate or it may be a cylindricalelement such as a sleeve which can be slid around a print cylinder of aprinting press. The support is a metal support such as aluminum orstainless steel. The metal can also be laminated to a plastic layer,e.g. polyester film.

A particularly preferred lithographic support is an electrochemicallygrained and anodized aluminum support. Graining and anodization ofaluminum is well known in the art. The anodized aluminum support may betreated to improve the hydrophilic properties of its surface. Forexample, the aluminum support may be silicated by treating its surfacewith a sodium silicate solution at elevated temperature, e.g. 95° C.Alternatively, a phosphate treatment may be applied which involvestreating the aluminum oxide surface with a phosphate solution that mayfurther contain an inorganic fluoride. Further, the aluminum oxidesurface may be rinsed with a citric acid or citrate solution. Thistreatment may be carried out at room temperature or may be carried outat a slightly elevated temperature of about 30 to 50° C. A furtherinteresting treatment involves rinsing the aluminum oxide surface with as bicarbonate solution. Still further, the aluminum oxide surface may betreated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid,phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid,polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinylalcohol, and acetals of polyvinyl alcohols formed by reaction with asulfonated aliphatic aldehyde. It is further evident that one or more ofthese post treatments may be carried out alone or in combination. Moredetailed descriptions of these treatments are given in GB-A-1 084 070,DE-A-4 423 140, DE-A-4 417 907, EP-A-659 909, EP-A-537 633, DE-A-4 001466, EP-A-292 801, EP-A-291 760 and U.S. Pat. No. 4,458,005.

The hydrophobic binder can be present in one or more layer(s) of thecoating. It is preferably an organic polymer which has acidic groupswith a pKa of less than 13 to ensure that the layer is soluble or atleast swellable in aqueous alkaline developers. Advantageously, thebinder is a polymer or polycondensate, for example a polyester,polyamide, polyurethane or polyurea. Polycondensates and polymers havingfree phenolic hydroxyl groups, as obtained, for example, by reactingphenol, resorcinol, a cresol, a xylenol or a trimethylphenol withaldehydes, especially formaldehyde, or ketones are also particularlysuitable. Condensates of sulfamoyl- or carbamoyl-substituted aromaticsand aldehydes or ketones are also suitable. Polymers ofbismethylol-substituted ureas, vinyl ethers, vinyl alcohols, vinylacetals or vinylamides and polymers of phenylacrylates and copolymers ofhydroxyl-phenylmaleimides are likewise suitable. Furthermore, polymershaving units of vinylaromatics, N-aryl(meth)acrylamides or aryl(meth)acrylates may be mentioned, it being possible for each of theseunits also to have one or more carboxyl groups, phenolic hydroxylgroups, sulfamoyl groups or carbamoyl groups. Specific examples includepolymers having units of 2-hydroxyphenyl (meth)acrylate, ofN-(4-hydroxyphenyl)(meth)acrylamide, ofN-(4-sulfamoylphenyl)-(meth)acrylamide, ofN-(4-hydroxy-3,5-dimethylbenzyl)-(meth)acrylamide, or 4-hydroxystyreneor of hydroxyphenylmaleimide. The polymers may additionally containunits of other monomers which have no acidic units. Such units includevinylaromatics, methyl(meth)acrylate, phenyl(meth)acrylate,benzyl(meth)acrylate, methacrylamide or acrylonitrile.

Any amount of binder can be used. The amount of the binder isadvantageously from 40 to 99.8% by weight, preferably from 70 to 99.4%by weight, particularly preferably from 80 to 99% by weight, based ineach case on the total weight of the nonvolatile components of thecoating. In a preferred embodiment, the polycondensate is a phenolicresin, such as a novolac, a resole or a polyvinylphenol. The novolac ispreferably a cresol/formaldehyde or a cresol/xylenol/formaldehydenovolac, the amount of novolac advantageously being at least 50% byweight, preferably at least 80% by weight, based in is each case on thetotal weight of all binders. Additional suitable polymeric binders aredescribed in EP-02102443, EP-02102444, EP-02102445, EP-02102446, filedon 15/10/2002, DE-A-4007428, DE-A-4027301 and DE-A-4445820.

A suitable negative-working alkaline developing printing plate comprisesa phenolic resin and a latent Brönsted acid which produces acid uponheating or IR radiation. These acids catalyze crosslinking of thecoating in a post-exposure heating step and thus hardening of theexposed regions. Accordingly, the non-exposed regions can be washed awayby a developer to reveal the hydrophilic substrate underneath. For amore detailed description of such a negative-working printing plateprecursor we refer to U.S. Pat. No. 6,255,042 and U.S. Pat. No.6,063,544 and to references cited in these documents.

In a preferred embodiment the lithographic printing plate precursor ofthe present invention is positive working and contains a hydrophilicsupport and a coating provided thereon comprising an infrared lightabsorbing dye and a hydrophobic binder which is soluble in an aqueousalkaline developer.

In the positive working embodiment, the dissolution behavior of thecoating in the developer can be fine-tuned by optional solubilityregulating components. More particularly, development accelerators anddevelopment inhibitors can be used. These ingredients can be added tothe layer(s) which comprise(s) the hydrophobic binder and/or to(an)other layer(s) of the coating.

Development accelerators are compounds which act as dissolutionpromoters because they are capable of increasing the dissolution rate ofthe coating. For example, cyclic acid anhydrides, phenols or organicacids can be used in order to improve the aqueous developability.Examples of the cyclic acid anhydride include phthalic anhydride,tetrahydrophthalic anhydride, hexahydrophthalic anhydride,3,6-endoxy-4-tetrahydro-phthalic anhydride, tetrachlorophthalicanhydride, maleic anhydride, chloromaleic anhydride, alpha -phenylmaleicanhydride, succinic anhydride, and pyromellitic anhydride, as describedin U.S. Pat. No. 4,115,128. Examples of the phenols include bisphenol A,p-nitrophenol, p-ethoxyphenol, 2,4,4′-trihydroxybenzophenone,2,3,4-trihydroxy-benzophenone, 4-hydroxybenzophenone,4,4′,4″-trihydroxy-triphenylmethane, and4,4′,3″,4″-tetrahydroxy-3,5,3′,5′-tetramethyltriphenyl-methane, and thelike. Examples of the organic acids include sulfonic acids, sulfonicacids, alkylsulfuric acids, phosphonic acids, phosphates, and carboxylicacids, as described in, for example, JP-A 60-88,942 and JP-A 2-96,755.Specific examples of these organic acids include p-toluenesulfonic acid,dodecylbenzenesulfonic acid, p-toluenesulfonic acid, ethylsulfuric acid,phenylphosphonic acid, phenylphosphinic acid, phenyl phosphate, diphenylphosphate, benzoic acid, isophthalic acid, adipic acid, p-toluic acid,3,4-dimethoxybenzoic acid, 3,4,5-trimethoxybenzoic acid,3,4,5-trimethoxycinnamic acid, phthalic acid, terephthalic acid,4-cyclohexene-1,2-dicarboxylic acid, erucic acid, lauric acid,n-undecanoic acid, and ascorbic acid. The amount of the cyclic acidanhydride, phenol, or organic acid contained in the coating ispreferably in the range of 0.05 to 20% by weight.

In a preferred embodiment, the coating also contains developerresistance means, also called development inhibitors, i.e. one or moreingredients which are capable of delaying the dissolution of theunexposed areas during processing. The dissolution inhibiting effect ispreferably reversed by heating, so that the dissolution of the exposedareas is not substantially delayed and a large dissolution differentialbetween exposed and unexposed areas can thereby be obtained. Suchdeveloper resistance means can be added to a layer comprising thehydrophobic binder or to another layer of the material.

The compounds described in e.g. EP-A 823 327 and WO97/39894 act asdissolution inhibitors due to interaction, e.g. by hydrogen bridgeformation, with the alkali-soluble binder(s) in the coating. Inhibitorsof this type typically comprise a hydrogen bridge forming group such asnitrogen atoms, onium groups, carbonyl (—CO—), sulfinyl (—SO—) orsulfonyl (—SO₂—) groups and a large hydrophobic moiety such as one ormore aromatic groups.

Other suitable inhibitors improve the developer resistance because theydelay the penetration of the aqueous alkaline developer into thecoating. Such compounds can be present in the layer(s) comprising thehydrophobic binder, as described in e.g. EP-A 950 518, and/or in adevelopment barrier layer on top of said layer, as described in e.g.EP-A 864 420, EP-A 950 517, WO 99/21725 and WO 01/45958. In the latterembodiment, the solubility of the barrier layer in the developer or thepenetrability of the barrier layer by the developer can be increased byexposure to heat or infrared light.

Preferred examples of inhibitors which delay the penetration of theaqueous alkaline developer into the coating include :

-   (a) A polymeric material which is insoluble in or impenetrable by    the developer, e.g. a hydrophobic (co-)polymer such as acrylic    polymers, polystyrene, styrene-acrylic copolymers, polyesters,    polyamides, polyureas, polyurethanes, nitrocellulosics and epoxy    resins; or water-repellent polymers such as polymers comprising    siloxane (silicones) and/or perfluoroalkyl units. The    water-repellent polymer may be present in an amount of e.g. between    0.5 and 15 mg/m², preferably between 0.5 and 10 mg/m², more    preferably between 0.5 and 5 mg/m² and most preferably between 0.5    and 2 mg/m² Higher or lower amounts are also suitable, depending on    the hydrophobic/oleophobic character of the compound. When the    water-repellent polymer is also ink-repelling, higher amounts than    15 mg/m² can result in poor ink-acceptance of the non-exposed areas.    An amount lower than 0.5 mg/m² on the other hand may lead to an    unsatisfactory development latitude: development of the exposed    areas is not completed.-   (b) Bifunctional compounds such as surfactants comprising a polar    group and a hydrophobic group such as a long chain hydrocarbon    group, a poly- or oligosiloxane and/or a perfluorinated hydrocarbon    group. A typical example is Megafac F-177, a perfluorinated    surfactant available from Dainippon Ink & Chemicals, Inc. A suitable    amount of such compounds is between 10 and 100 mg/m², more    preferably between 50 and 90 mg/m².-   (c) Bifunctional block-copolymers comprising a polar block such as a    poly- or oligocalkylene oxide) and a hydrophobic block such as a    long chain hydrocarbon group, a poly- or oligosiloxane and/or a    perfluorinated hydrocarbon group. A suitable amount of such    compounds is between 0.5 and 25 mg/m², preferably between 0.5 and 15    mg/m² and most preferably between 0.5 and 10 mg/m². A suitable    copolymer comprises about 15 to 25 siloxane units and 50 to 70    alkyleneoxide groups. Preferred examples include copolymers    comprising phenylmethylsiloxane and/or dimethylsiloxane as well as    ethylene oxide and/or propylene oxide, such as Tego Glide 410, Tego    Wet 265, Tego Protect 5001 or Silikophen P50/X, all commercially    available from Tego Chemie, Essen, Germany. Specific compounds are    the following:    wherein o, p, q, r and s are integers >1.

In formula A, a poly(alkylene oxide) block consisting of ethylene oxideand propylene oxide units is grafted to a polysiloxane block. In formulaB, long chain alcohols consisting of ethylene oxide and propylene oxideunits are grafted to a trisiloxane group.

The above mentioned poly- or oligosiloxane groups can be a linear,cyclic or complex cross-linked polymer or copolymer. The termpolysiloxane shall include any compound which contains more than onesiloxane group —Si(R,R′)—O—, wherein R and R′ are optionally substitutedalkyl or aryl groups. Preferred siloxanes are phenylalkylsiloxanes anddialkylsiloxanes. The number of siloxane groups in the polymer oroligomer is at least 2, preferably at least 10, more preferably at least20. It may be less than 100, preferably less than 60. The abovementioned perfluorinated hydrocarbon group includes e.g. —(CF₂)— units.The number of such units may be larger than 10, preferably larger than20. The poly- or oligo(alkylene oxide) block preferably includes unitsof the formula —C_(n)H_(2n)—O— wherein n is preferably an integer in therange 2 to 5. The moiety —C_(n)H_(2n)— may include straight or branchedchains. The alkylene moiety may also comprise optional substituents.Preferred embodiments and explicit examples of such polymers have beendisclosed in WO99/21725.

During coating and drying, the above mentioned inhibitor of type (b) and(c) tends to position itself, due to its bifunctional structure, at theinterface between the coating and air and thereby forms a separate toplayer even when applied as an ingredient of the coating solution of thelayer comprising the hydrophilic binder. Simultaneously, the surfactantsor bifunctional block-copolymers act as spreading agents which improvethe coating quality. The separate top layer thus formed is capable ofacting as the above mentioned barrier layer which delays the penetrationof the developer into the coating.

Alternatively, the inhibitor of type (a) to (c) can be applied in aseparate solution, coated on top of the layer(s) comprising thehydrophobic binder. In that embodiment, it may be advantageous to use asolvent in the second coating solution that is not capable of dissolvingthe ingredients present in the first layer so that a highly concentratedwater-repellent or hydrophobic phase is obtained at the top of thematerial which is capable of acting as the above mentioned developmentbarrier layer.

In the printing plate precursor of the present invention, the infraredlight absorbing dye may be present in the same layer(s) as thehydrophobic binder, in the optional barrier layer discussed above and/orin an optional other layer. According to a highly preferred embodiment,the dye is concentrated in or near the barrier layer, e.g. in anintermediate layer between the hydrophobic binder and the barrier layer.According to that embodiment, said intermediate layer comprises the IRabsorbing compound in an amount higher than the amount of IR absorbingcompound in the hydrophobic binder or in the barrier layer. Preferred IRabsorbing dyes are cyanine dyes, merocyanine dyes, indoaniline dyes,oxonol dyes, pyrilium dyes and squarilium dyes. Examples of suitable IRdyes are described in e.g. EP-A 823327, EP-A 978376, EP-A 1029667, EP-A1053868, EP-A 1093934; WO 97/39894 and WO 00/29214. A preferred compoundis the following cyanine dye:

The absoption spectra of the net reflection density versus wavelength(FIG. 1) of the coatings of this invention have been measured. Theabsorption maxima λmax (3) of the spectra are situated in the range from700 nm to 1000 nm, more specific in the range from 700 nm to 890 nm,most specific in the range from 700 nm to 850 nm. It has been found thatthe compositions of this invention (FIG. 1, solid line 1), which have aband width (4) measured at 80% of the net reflection density at λmaxlower than 1000 cm⁻¹, show no partial ablation after infrared exposure.The comparative compositions (FIG. 1, dotted line 2) having a band width(4) broader than 1000 cm⁻¹ show ablative side reactions after infraredexposure. Whilst the applicants do not wish to be limited by anytheoretical explanation of how their printing plate precursor operates,it is believed that in the comparative compositions aggregates of theinfrared light absorbing dye are formed which tend to form hot spots inthe coating leading to unwanted partial ablation.

To protect the surface of the coating, in particular from mechanicaldamage, a protective layer may also optionally be applied. Theprotective layer generally comprises at least one water-solublepolymeric binder, such as polyvinyl alcohol, polyvinylpyrrolidone,partially hydrolyzed polyvinyl acetates, gelatin, carbohydrates orhydroxyethylcellulose, and can be produced in any known manner such asfrom an aqueous solution or dispersion which may, if required, containsmall amounts, i.e. less than 5% by weight, based on the total weight ofthe coating solvents for the protective layer, of organic solvents. Thethickness of the protective layer can suitably be any amount,advantageously up to 5.0 μm, preferably from 0.1 to 3.0 μm, particularlypreferably from 0.15 to 1.0 μm.

Optionally, the coating and more specifically the layer(s) comprisingthe hydrophobic binder may further contain additional ingredients.Preferred ingredients are e.g. additional binders, especiallysulfonamide and phthalimide groups containing polymers, to improve therun length and chemical resistance of the plate. Examples of suchpolymers are those described in EP-A 933682, EP-A 894622 and WO99/63407.

Also colorants can be added such as dyes or pigments which provide avisible colour to the coating and which remain in the coating atunexposed areas so that a visible image is produced after exposure andprocessing. Typical examples of such contrast dyes are theamino-substituted tri- or diarylmethane dyes, e.g. crystal violet,methyl violet, victoria pure blue, flexoblau 630, basonylblau 640,auramine and malachite green. Also the dyes which are discussed in depthin the detailed description of EP-A 400706 are suitable as contrast dyesfor use in the printing plate precursor of the present invention.

Surfactants, especially perfluoro surfactants, silicon or titaniumdioxide particles, polymers particles such as matting agents and spacersare also well-known components of lithographic coatings which can beused in the plate precursor of the present invention.

For the preparation of the lithographic plate precursor, any knownmethod can be used. For example, the above ingredients can be dissolvedin a solvent mixture which does not react irreversibly with theingredients and which is preferably tailored to the intended coatingmethod, the layer thickness, the composition of the layer and the dryingconditions. Suitable solvents include ketones, such as methyl ethylketone (butanone), as well as chlorinated hydrocarbons, such astrichloroethylene or 1,1,1-trichloroethane, alcohols, such as methanol,ethanol or propanol, ethers, such as tetrahydrofuran, glycol-monoalkylethers, such as ethylene glycol monoalkyl ether, e.g.2-methoxy-1-propanol, or propylene glycol monoalkyl ether and esters,such as butyl acetate or propylene glycol monoalkyl ether acetate. It isalso possible to use a mixture which, for special purposes, mayadditionally contain solvents such as acetonitrile, dioxane,dimethylacetamide, dimethylsulfoxide or water.

Any coating method can be used for applying one or more coatingsolutions to the hydrophilic surface of the support. A multi-layercoating can be applied by coating/drying each layer consecutively or bythe simultaneous coating of several coating solutions at once. In thedrying step, the volatile solvents are removed from the coating untilthe coating is self-supporting and dry to the touch. However it is notnecessary (and may not even be possible) to remove all the solvent inthe drying step. Indeed the residual solvent content may be regarded asan additional composition variable by means of which the composition maybe optimised. Drying is typically carried out by blowing hot air ontothe coating, typically at a temperature of at least 70° C., suitably80-150° C. and especially 90-140° C. Also infrared lamps can be used.The drying time may typically be 15-600 seconds.

The material can be image-wise exposed directly with heat, e.g. by meansof a thermal head, or indirectly by infrared light, preferably nearinfrared light. The infrared light is preferably converted into heat byan IR light absorbing compound as discussed above. The heat-sensitivelithographic printing plate precursor of the present invention ispreferably not sensitive to visible light, i.e. no substantial effect onthe dissolution rate of the coating in the developer is induced byexposure to visible light. Most preferably, the coating is not sensitiveto ambient daylight, i.e. visible (400-750 nm) and near UV light(300-400 nm) at an intensity and exposure time corresponding to normalworking conditions so that the material can be handled without the needfor a safe light environment. “Not sensitive” to daylight shall meanthat no substantial change of the dissolution rate of the coating in thedeveloper is induced by exposure to ambient daylight. In a preferreddaylight stable embodiment, the coating does not comprise photosensitiveingredients, such as (quinone)diazide or diazo(nium) compounds,photoacids, photoinitiators, sensitizers etc., which absorb the near UVand/or visible light that is present in sun light or office lighting andthereby change the solubility of the coating in exposed areas.

The printing plate precursor of the present invention can be exposed tolight, e.g. by means of LEDs or a laser head. Preferably, one or morelasers or a laser diode are used. The light used for the exposure isinfrared light having a wavelength in the range of λmax±20 nm, morespecific in the range of λmax±10 nm; most specific in the range ofλmax±5 nm. Preferably a laser such as a semiconductor laser diode isused. The required laser power depends on the sensitivity of theimage-recording layer, the pixel dwell time of the laser beam, which isdetermined by the spot diameter (typical value of modern plate-settersat 1/e² of maximum intensity: 10-25 μm), the scan speed and theresolution of the exposure apparatus (i.e. the number of addressablepixels per unit of linear distance, often expressed in dots per inch ordpi; typical value: 1000-4000 dpi).

Two types of laser-exposure apparatuses are commonly used: internal(ITD) and external drum (XTD) plate-setters. ITD plate-setters forthermal plates are typically characterized by a very high scan speed upto 1500 m/sec and may require a laser power of several Watts. The AgfaGalileo T (trademark of Agfa Gevaert N.V.) is a typical example of aplate-setter using the ITD-technology. XTD plate-setters operate at alower scan speed typically from 0.1 m/sec to 20 m/sec and have a typicallaser-output-power per beam from 20 mW up to 500 mW. The CreoTrendsetter plate-setter family (trademark of Creo) and the AgfaExcalibur plate-setter family (trademark of Agfa Gevaert N.V.) both makeuse of the XTD-technology. The known plate-setters can be used as anoff-press exposure apparatus, which offers the benefit of reduced pressdown-time. XTD plate-setter configurations can also be used for on-pressexposure, offering the benefit of immediate registration in amulti-color press. More technical details of on-press exposureapparatuses are described in e.g. U.S. Pat. No. 5,174,205 and U.S. Pat.No. 5,163,368. In the development step, the non-image areas of thecoating are removed by immersion in a conventional aqueous alkalinedeveloper, which may be combined with mechanical rubbing, e.g. by arotating brush. During development, any water-soluble protective layerpresent is also removed. Silicate-based developers which have a ratio ofsilicon dioxide to alkali metal oxide of at least 1 are preferred toensure that the alumina layer (if present) of the substrate is notdamaged. Preferred alkali metal oxides include Na₂O and K₂O, andmixtures thereof. In addition to alkali metal silicates, the developermay optionally contain further components, such as buffer substances,complexing agents, antifoams, organic solvents in small amounts,corrosion inhibitors, dyes, surfactants and/or hydrotropic agents aswell known in the art. The development is preferably carried out attemperatures from 20 to 40° C. in automated processing units ascustomary in the art. For regeneration, alkali metal silicate solutionshaving alkali metal contents of from 0.6 to 2.0 mol/l can suitably beused. These solutions may have the same silica/alkali metal oxide ratioas the developer (generally, however, it is lower) and likewiseoptionally contain further additives. The required amounts ofregenerated material must be tailored to the developing apparatusesused, daily plate throughputs, image areas, etc. and are in general from1 to 100 ml per square meter of recording material. The addition can beregulated, for example, by measuring the conductivity as described inEP-A 0 556 690.

The plate precursor according to the invention can, if required, then bepost-treated with a suitable correcting agent or preservative as knownin the art. To increase the resistance of the finished printing plateand hence to extend the print run, the layer can be briefly heated toelevated temperatures (“baking”). As a result, the resistance of theprinting plate to washout agents, correction agents and UV-curableprinting inks also increases. Such a thermal post-treatment isdescribed, inter alia, in DE-A 14 47 963 and GB-A 1 154 749.

Besides the mentioned post-treatment, the processing of the plateprecursor may also comprise a rinsing step, a drying step and/or agumming step.

The printing plate thus obtained can be used for conventional, so-calledwet offset printing, in which ink and an aqueous dampening liquid issupplied to the plate. Another suitable printing method uses so-calledsingle-fluid ink without a dampening liquid. Single-fluid inks which aresuitable for use in the method of the present invention have beendescribed in U.S. Pat. No. 4,045,232; U.S. Pat. No. 4,981,517 and U.S.Pat. No. 6,140,392. In a most preferred embodiment, the single-fluid inkcomprises an ink phase, also called the hydrophobic or oleophilic phase,and a polyol phase as described in WO 00/32705.

EXAMPLES

Preparation of the Lithographic Support

A 0.30 mm thick aluminum foil was degreased by immersing the foil in anaqueous solution containing 5 g/l of sodium hydroxide at 50° C. andrinsed with demineralized water. The foil was then electrochemicallygrained using an alternating current in an aqueous solution containing 4g/l of hydrochloric acid, 4 g/l of hydroboric acid and 5 g/l of aluminumions at a temperature of 35° C. and a current density of 1200 A/m² toform a surface topography with an average center-line roughness Ra of0.5 μm. After rinsing with demineralized water, the aluminum foil wasthen etched with an aqueous solution containing 300 g/l of sulfuric acidat 60° C. for 180 seconds and rinsed with demineralized water at 25° C.for 30 seconds. The foil was subsequently subjected to anodic oxidationin an aqueous solution containing 200 g/l of sulfuric acid at atemperature of 45° C., a voltage of 10V at a current density of 150 A/m²during 300 seconds to form an anodic oxidation film of 3 g/m² of Al₂O₃,then washed with demineralized water, post-treated with a solutioncontaining 4 g/l polyvinylphosphonic acid and subsequently with analuminium trichloride solution. Finally, the foil was rinsed withdemineralized water at 20° C. and dried.

The printing plate precursors of the Examples 1-5 (invention) andExamples 6-7 (comparative) were produced by coating the solutionsdefined in Table 1 onto the above described lithographic support. Thecoating solutions were applied at a wet coating thickness of 26 μm on acoating line operating at a speed of 10.8 m/min and then dried at 135°C. TABLE 1 composition of the coating solutions Ingredients (g) Ex. 1(inv.) Ex. 2 (inv.) Ex. 3 (inv.) Ex. 4 (inv.) Ex. 5 (inv.) Ex. 6 (comp.)Ex. 7 (comp.) Tetrahydrofuran 209.0 209.0 209.0 209.0 209.0 209.0 209.0Alnovol SPN452 (1) 103.5 103.5 103.5 103.5 103.5 103.5 103.5 Dowanol PM(2) 385.4 385.4 385.4 385.4 385.4 385.4 385.4 Methyl ethyl ketone 265.9265.9 265.9 265.9 265.9 265.9 265.9 S0094 (3) 0.43 0.64 0.86 0.107 1.281.71 2.14 Basonyl Blue 640 (4) 0.53 0.53 0.53 0.53 0.53 0.53 0.53 TegoGlide 410 (5) 21.35 8.50 8.50 8.50 8.50 8.50 8.50 Tego Wet 265 (5) 8.5321.55 21.55 21.55 21.55 21.55 21.55 2,3,4-trimethoxy 5.34 5.34 5.34 5.345.34 5.34 5.34 cinnamic acid(1) Alnovol SPN452 is a 40.5 wt. % solution of novolac in Dowanol PM(trademark of Clariant).(2) 1-methoxy-2-propanol from Dow Chemical Company.(3) S0094 is an IR absorbing cyanine dye trademark of FEW Chemicals.S0094 has the chemical structure IR-1 shown above.(4) Basonyl Blue 640 is a quaternized triarylmethane dye trademark ofBASF.(5) Tego Wet 265 and Tego Glide 410 are both block-copolymers comprisingpolysiloxanes, trademarksof Tego Chemie Service GmbH.; 1 wt % solutionin Dowanol PM.Evaluation and Results

The infrared absorption spectra of the coating of each of the aboveprinting plate precursors were measured with a Perkin Elmer Lambda 900spectrophotometer in mixed reflection mode. The net reflection densityof the coating was obtained using a non-coated sample, obtained bywashing the coating from the support with methyl ethyl ketone, as areference. The band width at 80% of the absorption peak was measured asexplained above with reference to FIG. 1. The values are given in Table2 given below (heading “BW80”).

Each of the above printing plate precursors was exposed on a prototypeXTD infrared diode laser (830 nm) image-setter at varying powerdensities as indicated in Table 2 given below. The extent of ablation ofthe exposed coating was evaluated by comparing the SEM-images obtainedof the exposed samples with standard SEM-images (FIGS. 2 to 6). Thestandard SEM-images (FIGS. 2 to 6) were obtained by exposing a prior artmaterial with the same image-setter as used for Examples 1-7 of thisinvention. The power density of the exposure increases from FIG. 2 (lowvalue) to FIG. 6 (high value). The visual evaluation of the obtainedimages was quantified on a scale from 1 to 5 as follows:

-   “1”=the coating is defect-free and no ablation debris is deposited    on the surface of the coating; see FIG. 2.-   “2”=onset of damage to the coating (a few small holes are visible)    but no ablation debris is observed; see FIG. 3.-   “3”=occurrence of a significant amount of holes in the coating but    no bubbles nor ablation debris detectable on the surface of the    coating; see FIG. 4.-   “4”=onset of deposition of ablation debris on the surface of the    coating; the coating shows many defects (holes, bubbles); see FIG.    5.-   “5”=a significant amount of ablation debris is deposited on the    surface of the coating, which is substantially damaged by the    exposure; see FIG. 6.

The qualification on the basis of SEM images corresponds well with thevisual perception of dust on the imaged plates. Plates which arequalified on the basis of the SEM images as grade “4” or “5”, show adeposition of dust which is perceptible to a human observer and whichcan be wiped off with a cloth or paper tissue. At grade “4”, the dust isonly visible when looking at the reflection of a light source (e.g. awindow) on the surface at low angles while a qualification of “5”corresponds to an amount of dust which is very clearly visible on theplate's surface at any angle. Below “3”, no dust is perceivable visuallyand neither detectable on the SEM images. TABLE 2 extent of ablation ona scale 1-5 upon IR exposure at various power densities Example BW80Power density (kW/cm²) no. (cm⁻¹) 145 174 204 233 263 292 1 (inv.) 616 11 1 2 2 2 2 (inv.) 675 1 1 2 2 2 2 3 (inv.) 751 2 2 2 3 3 3 4 (inv.) 8292 2 3 3 3 3 5 (inv.) 984 2 3 3 3 3 3 6 (comp.) 1586 2 2 2 3 4 4 7(comp.) 1736 2 2 3 3 4 5

The results in Table 2 demonstrate that Examples 1-5, which have a bandwidth at 80% of the infrared absorption peak lower than 1000 cm⁻¹, canbe exposed at a high power density without the occurrence of ablationdebris (grade 1 to 3). Comparative Examples 6 and 7 generate ablationdebris (grade 4 or 5) upon infrared exposure above 233 kW/cm².

1. A heat-sensitive lithographic printing plate precursor comprising (i)a metal support having a hydrophilic surface or provided with ahydrophilic layer and (ii) provided thereon a coating comprising aninfrared light absorbing dye and a hydrophobic binder which is solublein an aqueous alkaline developer, wherein the coating has a lightabsorption spectrum (1) of net reflection density versus wavelengthwhich has an absorption peak (3) at a wavelength λmax in the rangebetween 700 and 1000 nm, wherein said absorption peak has a band width(4), defined as the wave number interval at 80% of the net reflectiondensity at λmax, which is lower than 1000 cm⁻¹.
 2. A printing plateprecursor according to claim 1 wherein ζmax of the light absorptionspectrum of the coating ranges between 700 nm and 890 nm.
 3. A printingplate precursor according to claim 1 wherein λmax of the lightabsorption spectrum of the coating ranges between 700 nm and 850 nm. 4.A printing plate precursor according to claim 1 wherein the coating iscapable of dissolving in an aqueous alkaline developer at a lowerdissolution rate in areas of the coating which are exposed to infraredlight than in unexposed areas.
 5. A printing plate precursor accordingto claim 1 wherein the coating is capable of dissolving in an aqueousalkaline developer at a higher dissolution rate in areas of the coatingwhich are exposed to infrared light than in unexposed areas.
 6. Aprinting plate precursor according to claim 5 wherein the hydrophobicbinder is a phenolic resin and wherein the coating further comprises adissolution inhibitor which is selected from the group consisting of (a)an organic compound comprising an aromatic group and a hydrogen bondingsite, (b) a hydrophobic or water-repellent polymer which is insoluble inor impenetrable by the developer, (c) a surfactant comprising a polargroup and a hydrophobic group or (d) a block-copolymer comprising apoly- or oligo(alkylene oxide) block and a hydrophobic block.
 7. Aprinting plate precursor according to claim 1 wherein the infrared dyeis selected from the group consisting of cyanine dyes, merocyanine dyes,indoaniline dyes, oxonol dyes, pyrilium dyes and squarilium dyes.
 8. Aprinting plate precursor according to claim 1 wherein the infrared dyehas the following structure:


9. A printing plate precursor according to claim 6 wherein the amount ofthe water-repellent polymer in the coating is between 0.5 and 15 mg/m².10. A printing plate precursor according to claim 6 wherein the amountof the surfactant in the coating is between 10 and 100 mg/m².
 11. Aprinting plate precursor according to claim 6 wherein the amount of theblock-copolymer in the coating is between 0.5 and 25 mg/m².
 12. A methodof exposing a lithographic printing plate precursor according to claim 1wherein the coating does not generate ablation upon exposure to laserlight having a wavelength in the range of λmax±20 nm and a power densityabove 233 kW/cm².
 13. A printing plate precursor according to claim 2wherein the coating is capable of dissolving in an aqueous alkalinedeveloper at a lower dissolution rate in areas of the coating which areexposed to infrared light than in unexposed areas.
 14. A printing plateprecursor according to claim 3 wherein the coating is capable ofdissolving in an aqueous alkaline developer at a lower dissolution ratein areas of the coating which are exposed to infrared light than inunexposed areas.
 15. A printing plate precursor according to claim 2wherein the coating is capable of dissolving in an aqueous alkalinedeveloper at a higher dissolution rate in areas of the coating which areexposed to infrared light than in unexposed areas.
 16. A printing plateprecursor according to claim 3 wherein the coating is capable ofdissolving in an aqueous alkaline developer at a higher dissolution ratein areas of the coating which are exposed to infrared light than inunexposed areas.
 17. A printing plate precursor according to claim 2wherein the infrared dye is selected from the group consisting ofcyanine dyes, merocyanine dyes, indoaniline dyes, oxonol dyes, pyriliumdyes and squarilium dyes.
 18. A printing plate precursor according toclaim 3 wherein the infrared dye is selected from the group consistingof cyanine dyes, merocyanine dyes, indoaniline dyes, oxonol dyes,pyrilium dyes and squarilium dyes.
 19. A printing plate precursoraccording to claim 4 wherein the infrared dye is selected from the groupconsisting of cyanine dyes, merocyanine dyes, indoaniline dyes, oxonoldyes, pyrilium dyes and squarilium dyes.
 20. A printing plate precursoraccording to claim 2 wherein the infrared dye has the followingstructure:


21. A printing plate precursor according to claim 3 wherein the infrareddye has the following structure: